1. Field of the Invention and Related Art Statement
This invention relates to a sector mark detecting apparatus in an optical disc apparatus detecting a starting point of each of a plurality of sectors formed in each track in an optical disc.
A disc-like recording medium is extensively used in an optical recording and reading apparatus.
In a recording and reading apparatus (which shall be mentioned as an optical disc apparatus hereinafter) using the above mentioned disc-like reading medium (which shall be mentioned as an optical disc hereinafter), information data is recorded along a concentric circular or spiral track of an optical disc. In such a case, a sector formed by dividing each track into many sectors is used as a data processing unit.
That is, when recording or reading information in an optical disc, in controlling a random access or READ/WRITE, a mark showing the starting point is attached as one recording unit. This recording unit is called a sector and this mark is called a sector mark.
The above mentioned sector mark makes easy not only the above mentioned control but also the timing control at the time of reading and writing data and is useful to elevate the reliability of the detection of a synchronous signal.
As mentioned above, the sector mark is effective to control an access or to detect a signal at the time of reading or writing and therefore its detection reliability must be high enough. In an optical disc, it is difficult to make a recording film defect or noise small enough. It is necessary to make an apparatus which can allow an error as a bit error rate of 10.sup.-5 to 10.sup.-6. Among the above mentioned errors, a burst error rather than a random error is particularly a problem.
The reliability of recorded data will be improved by adding an error correcting mark. A burst error can be dispersed by an interleaving means of dispersing and recording data. Even a considerably long burst error can be taken care of.
However, this means can not be applied to the sector mark detection which can not be practiced without any reliability elevation.
In order to solve the above mentioned problem, a sector mark detecting means wherein a special pattern is made a sector mark pattern is disclosed in the publication of a Japanese Patent Application Laid Open No.169337/1983.
Also, the sector mark pattern is pre-formatted in the disc together with the ID part having information such as the track and sector address.
This sector mark is very high in detection probability as described in the above mentioned Japanese Patent Application Laid Open No.169337/1983 but, in fact, may not be often detected due to the stain of the disc, the flaw at the time of molding the disc and the tracking lag.
Therefore, in a prior art example in a Japanese Patent Application Laid Open No.5476/1986, a means such a gate is used to prevent the mis-detection of sector marks. However, the larger the rotation error, the larger the gate width must be. In considering that the mark may not be continuously detected, the gate width must be larger. Thus, the mis-detection (by the detection with .sub.5 C.sub.3 or the like) near a sector mark can not be prevented.
In a prior art example disclosed in a Japanese Patent Application Laid Open No.201573/1985, there is adopted a method wherein a sector mark is indicated by a period timer by which a sector mark is to be detected inherently by utilizing that the sector mark should be detected in an equal period and, in case a sector mark is not detected even at the end of this period, a quasi-sector mark signal will be generated and will be used in place of the inherent sector mark.
In the system disclosed in the above mentioned Japanese Patent Application Laid Open No.201573/1985, after it is detected that a sector mark reading signal has not been generated, a quasi-sector mark signal will be generated and therefore the quasi-sector mark signal can not help being generated by all means as delayed from the timing when the sector mark reading signal should be inherently generated.
There has been a defect that, if the above mentioned timing lags, the reliability on the inherent sector mark reading will be reduced.